Definition of downer in English:

noun

So-called benzos - benzodiazepines include prescription downers such as Valium and Xanax - are commonly mixed with methadone.

It should never be used at the same time as any other depressant such as downers, alcohol, GHB, or sedatives.

People with bipolar disorder often go undiagnosed for years; many end up self-medicating with booze or illicit drugs - downers mostly, because they help control, if you can call it that, the manic swings of the disorder.